The main task of a hermetic, i.e. fluid-tight, pressure sensor is to measure pressure in harsh or dangerous environmental conditions. A first important reason to have a hermetic sensor is that the environment can damage the elements of the sensor which are enclosed in the housing of the sensor. Damage could make the sensor malfunction. A second important reason is to have a constant internal pressure which is important for low pressure sensing accuracy. In practice, a sensor housing is considered hermetic when during a Helium leak test, the leakage does not exceed the limit of approximately 5×10−8 mBar×l/sec He.
A pressure sensor is known from U.S. Pat. No. 6,763,724 B2. This pressure sensor has a tubular port fitting and a tubular housing, which results in the design being relatively long. A flange of the housing and the port fitting are welded to a support flange forming a hermetic seal. A compressive force on an O-ring forms a liquid-tight seal and maintains an axial load on the entire stack of components in the housing. Consequently, the housing is not gas-tight and the electrical elements are not hermetical sealed in the housing.
For transmission applications, there is a need for smaller, fully hermetic sensors. This means that the sensor should have less height. Furthermore, electronics of the sensor need to be sealed from the measured media, in particular to prevent possible copper-sulfur corrosion.
Smaller pressure sensors are known from U.S. Publication No. 20090282926 A1 and U.S. Pat. No. 8,516,897 B1. These sensors comprise a hermetic housing for their sensor elements. They use a sensor chip having a glass base which is disposed in a hermetically sealed housing. A diaphragm is formed in the sensor chip. The glass base is fused to the housing to form a glass to metal seal. A glass to metal seal is prone to failure in at least two cases: 1) a lack of a hermetic connection (e.g., due to a fault in the soldering process); and 2) glass is a brittle material that may crack when pressure peaks and dynamic forces are applied. As the fluid pressure is acting directly on the sensor chip, the glass base, and the connections (e.g., glass-silicon connection and glass-metal connection), there is a potential risk that the pressure sensor will malfunction due to damage of the glass base or any of the glass-connections.